NAME

ng_tcpmss — netgraph node to adjust TCP MSS option

SYNOPSIS

#include < netgraph.h>
#include < netgraph/ng_tcpmss.h>

DESCRIPTION

The tcpmss node type is designed to alter the Maximum Segment Size option of TCP packets. This node accepts an arbitrary number of hooks. Initially a new hook is considered unconfigured. The NG_TCPMSS_CONFIG control message is used to configure a hook.

CONTROL MESSAGES

This node type supports the generic control messages, plus the following.

NGM_TCPMSS_CONFIG ( config)

This control message configures node to do given MSS adjusting on a particular hook. It requires the struct ng_tcpmss_config to be supplied as an argument:

struct ng_tcpmss_config { 
 char  inHook[NG_HOOKSIZ]; 
 char  outHook[NG_HOOKSIZ]; 
 uint16_t maxMSS; 
}

This means: packets received on inHook would be checked for TCP MSS option and the latter would be reduced down to maxMSS if it exceeds maxMSS. After that, packets would be sent to hook outHook.

NGM_TCPMSS_GET_STATS ( getstats)

This control message obtains statistics for a given hook. The statistics are returned in struct ng_tcpmss_hookstat:

struct ng_tcpmss_hookstat { 
 uint64_t Octets;  /* total bytes */ 
 uint64_t Packets; /* total packets */ 
 uint16_t maxMSS;  /* maximum MSS */ 
 uint64_t SYNPkts; /* TCP SYN packets */ 
 uint64_t FixedPkts; /* changed packets */ 
};

NGM_TCPMSS_CLR_STATS ( clrstats)

This control message clears statistics for a given hook.

NGM_TCPMSS_GETCLR_STATS ( getclrstats)

This control message obtains and clears statistics for a given hook.

EXAMPLES

In the following example, packets are injected into the tcpmss node using the ng_ipfw(4) node.

# Create tcpmss node and connect it to ng_ipfw node 
ngctl mkpeer ipfw: tcpmss 100 qqq 
 
# Adjust MSS to 1452 
ngctl msg ipfw:100 config '{ inHook="qqq" outHook="qqq" maxMSS=1452 }' 
 
# Divert traffic into tcpmss node 
ipfw add 300 netgraph 100 tcp from any to any tcpflags syn out via fxp0 
 
# Let packets continue with ipfw after being hacked 
sysctl net.inet.ip.fw.one_pass=0

SHUTDOWN

This node shuts down upon receipt of an NGM_SHUTDOWN control message, or when all hooks have been disconnected.

SEE ALSO

netgraph(4), ng_ipfw(4)

HISTORY

The ng_tcpmss node type was implemented in FreeBSD 6.0.

AUTHORS

Alexey Popov <lollypop@flexuser.ru> Gleb Smirnoff <glebius@FreeBSD.org>

BUGS

When running on SMP, system statistics may be broken.